Method of manufacturing nitride semiconductor device

ABSTRACT

A method of manufacturing a nitride semiconductor device includes: forming, on or above a p-type nitride semiconductor tunnel junction layer, a first n-type nitride semiconductor layer that forms a tunnel junction with the p-type nitride semiconductor tunnel junction layer, the first n-type nitride semiconductor layer having a first impurity concentration and a first thickness; forming, on or above the first n-type nitride semiconductor layer, in a nitrogen atmosphere, a second n-type nitride semiconductor layer having a second n-type impurity concentration less than the first n-type impurity concentration and a second thickness; and forming, on or above the second n-type nitride semiconductor layer, in a hydrogen atmosphere, a third n-type nitride semiconductor layer having a third n-type impurity concentration less than the first n-type impurity concentration and a third thickness.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims priority to Japanese PatentApplication No. 2020-059321, filed on Mar. 30, 2020, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to a method of manufacturing a nitridesemiconductor device.

Japanese Patent Publication Nos. 2017-157667 and 2008-130877 describenitride semiconductor light emitting devices that include a tunneljunction layer. A tunnel junction layer includes a p-type semiconductorlayer with a relatively high p-type impurity concentration and an n-typesemiconductor layer with a relatively high n-type impurityconcentration. In Japanese Patent Publication No. 2017-157667, as a partof a tunnel junction layer, an n⁺⁺-GaN layer having a Si concentrationof 4×10²⁰ cm⁻³ or more is formed using an N₂ carrier gas, then an n-GaNlayer having a Si concentration of 8×10¹⁸ cm⁻³ is formed using a H₂carrier gas, and subsequently an n-GaN contact layer is formed. InJapanese Patent Publication No. 2008-130877, an n-typeIn_(0.25)Ga_(0.75)N tunnel junction layer having a Si concentration of1×10²⁰ cm⁻³ is formed using a nitrogen carrier gas, then an n-type GaNevaporation suppression layer having a Si concentration of 1×10²⁰ cm⁻³is formed, and then an n-type GaN layer having a Si concentration of1×10¹⁹ cm⁻³ is formed using a hydrogen carrier gas.

SUMMARY

One object of the present disclosure is to provide a nitridesemiconductor device with enhanced reliability.

According to one aspect of the present disclosure, a method ofmanufacturing a nitride semiconductor device includes: forming, on orabove a p-type nitride semiconductor tunnel junction layer, a firstn-type nitride semiconductor layer that forms a tunnel junction with thep-type nitride semiconductor tunnel junction layer, first n-type nitridesemiconductor layer having a first impurity concentration and a firstthickness; forming, on or above the first n-type nitride semiconductorlayer, in a nitrogen atmosphere, a second n-type nitride semiconductorlayer having a second n-type impurity concentration less than the firstn-type impurity concentration and a second thickness; and forming, on orabove the second n-type nitride semiconductor layer, in a hydrogenatmosphere, a third n-type nitride semiconductor layer having a thirdn-type impurity concentration less than the first n-type impurityconcentration and a third thickness.

According to certain embodiments of the manufacturing method of thepresent disclosure, a nitride semiconductor device with enhancedreliability can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a nitride semiconductordevice according to an embodiment of the present disclosure;

FIG. 2 is a flowchart illustrating a method of manufacturing a nitridesemiconductor device according to an embodiment of the presentdisclosure;

FIG. 3A is a schematic cross-sectional view illustrating the method ofmanufacturing a nitride semiconductor device according to the embodimentof the present disclosure;

FIG. 3B is a schematic cross-sectional view illustrating the method ofmanufacturing a nitride semiconductor device according to the embodimentof the present disclosure;

FIG. 3C is a schematic cross-sectional view illustrating the method ofmanufacturing a nitride semiconductor device according to the embodimentof the present disclosure;

FIG. 3D is a schematic cross-sectional view illustrating the method ofmanufacturing a nitride semiconductor device according to the embodimentof the present disclosure;

FIG. 3E is a schematic cross-sectional view illustrating the method ofmanufacturing a nitride semiconductor device according to the embodimentof the present disclosure;

FIG. 3F is a schematic cross-sectional view illustrating the method ofmanufacturing a nitride semiconductor device according to the embodimentof the present disclosure;

FIG. 4 is a schematic cross-sectional view of a nitride semiconductordevice according to a modified example of the present disclosure;

FIG. 5 is a flowchart illustrating a method of manufacturing a nitridesemiconductor device according to the modified example of the presentdisclosure;

FIG. 6 illustrates an atomic force microscope-taken observation image ofan experimental example; and

FIG. 7 illustrates an atomic force microscope-taken observation image ofa comparative example.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings. It should be noted that in each drawing,the same elements are denoted by the same reference numerals.

FIG. 1 is a schematic cross-sectional view of a nitride semiconductordevice 1 according to an embodiment of the present disclosure. Thenitride semiconductor device 1 illustrated in FIG. 1 is a light emittingdevice. The nitride semiconductor device 1 of the present embodimentincludes a p-type nitride semiconductor tunnel junction layer 43, afirst n-type nitride semiconductor layer 51, a second n-type nitridesemiconductor layer 52, and a third n-type nitride semiconductor layer53. The nitride semiconductor device 1 can include a substrate 10, afirst layered portion 20, a light emitting layer 30, a second layeredportion 40, a third layered portion 50, an n-side electrode 81, and ap-side electrode 82.

Examples of the material of the substrate 10 include sapphire, silicon,SiC, GaN, and the like. A buffer layer may be provided between thesubstrate 10 and the first layered portion 20.

A semiconductor layered body is a layered body in which a plurality ofsemiconductor layers made of nitride semiconductors are layered, andincludes the first layered portion 20, the light emitting layer 30, thesecond layered portion 40, and the third layered portion 50. A nitridesemiconductor may include a semiconductor having any compositionobtained by changing the composition ratio of x and y in a chemicalformula of In_(x) Al_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1, x+y≤1) within theirrespective ranges. In the semiconductor layered body, the first layeredportion 20, the light emitting layer 30, the second layered portion 40,and the third layered portion 50 are arranged in this order from theside of the substrate 10.

The first layered portion 20 includes one or more n-type nitridesemiconductor layers. An n-type nitride semiconductor layer may be alayer made of a nitride semiconductor containing an n-type impurity suchas silicon (Si) or germanium (Ge). A nitride semiconductor constitutingthe n-type nitride semiconductor layer may be, for example, GaN and mayinclude indium (In) and/or aluminum (Al). For example, the n-typeimpurity concentration (Si concentration) of the n-type nitridesemiconductor layer containing Si as an n-type impurity is 1×10¹⁸/cm³ ormore and 1×10²⁰/cm³ or less. The first layered portion 20 may include anundoped layer. The undoped layer is a layer in which an n-type impurityand a p-type impurity are intentionally not doped. The concentrations ofan n-type impurity and a p-type impurity in the undoped layer are, forexample, are concentrations not exceeding the detection limit in aresult of analysis such as secondary ion mass spectrometry (SIMS). In acase in which an undoped layer is adjacent to a layer intentionallydoped with an n-type impurity and/or a p-type impurity, the undopedlayer may contain an n-type impurity and/or a p-type impurity due todiffusion from the adjacent layer or the like.

The first layered portion 20 can include, for example, a GaN layer 21,an n-type GaN layer 22, and a multilayer film 23. The GaN layer 21 is anundoped layer. The thickness of the GaN layer 21 can be 2 μm or more and5 μm or less. The GaN layer 21 may be omitted. The n-type GaN layer 22includes Si as an n-type impurity. The n-type impurity concentration ofthe n-type GaN layer 22 can be 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ orless. The thickness of the n-type GaN layer 22 can be 3 μm or more and 7μm or less. The multilayer film 23 is a film in which a plurality ofpairs of an undoped GaN layer and an undoped InGaN layer are layered.The total thickness of the multilayer film 23 can be 30 nm or more and300 nm or less.

The light emitting layer 30 is provided between the first layeredportion 20 and the second layered portion 40. For example, the lightemitting layer 30 has a multi-quantum well structure including aplurality of well layers and a plurality of barrier layers. For theplurality of well layers, InGaN is used, for example. For the pluralityof barrier layers, GaN is used, for example. The light emitting layer 30may be an undoped layer in its entirety or at least a portion thereofmay contain an n-type impurity and/or a p-type impurity. The lightemitted by the light emitting layer 30 may be ultraviolet light orvisible light. The light emitting layer 30 can emit, for example, bluelight (peak wavelength of 430 nm or more and 490 nm or less).

The second layered portion 40 includes one or more p-type nitridesemiconductor layers. A p-type nitride semiconductor layer may be alayer made of a nitride semiconductor containing a p-type impurity suchas magnesium (Mg). A nitride semiconductor constituting the p-typenitride semiconductor layer may be, for example, GaN and may include Inand/or Al. For example, the p-type impurity concentration (Mgconcentration) of the p-type nitride semiconductor layer containing Mgas a p-type impurity is 5×10¹⁹/cm³ or more and 5×10²⁰/cm³ or less. Thesecond layered portion 40 may include an undoped layer.

The second layered portion 40 can include, for example, a p-type AlGaNlayer 41, a GaN layer 42, and a p-type nitride semiconductor tunneljunction layer 43. The p-type AlGaN layer 41 includes Mg as a p-typeimpurity. The p-type impurity concentration of the p-type AlGaN layer 41can be 5×10¹⁹/cm³ or more and 5×10²⁰/cm³ or less. The thickness of thep-type AlGaN layer 41 can be 2 nm or more and 20 nm or less. The GaNlayer 42 is an undoped layer. The thickness of the GaN layer 42 can be20 nm or more and 200 nm or less. By providing, between the p-typenitride semiconductor tunnel junction layer 43 and the p-type nitridesemiconductor layer below the p-type nitride semiconductor tunneljunction layer 43, a high resistance layer, such as the GaN layer 42, ofwhich the resistance is higher than that of the p-type nitridesemiconductor tunnel junction layer 43 and that of the p-type nitridesemiconductor layer, the diffusion of current in the directionintersecting the layered direction of the second layered portion 40(horizontal direction in FIG. 1) can be promoted. The thickness of ahigh resistance layer, such as the GaN layer 42, can be made greaterthan the thickness of the p-type nitride semiconductor tunnel junctionlayer 43.

The p-type nitride semiconductor tunnel junction layer 43 includes ap-type impurity. The p-type nitride semiconductor tunnel junction layer43 is, for example, a p-type GaN layer containing Mg as a p-typeimpurity. The p-type impurity concentration and the thickness of thep-type nitride semiconductor tunnel junction layer 43 are set to be ableto form a tunnel junction with the first n-type nitride semiconductorlayer 51, as will be described later below.

The third layered portion 50 includes a first n-type nitridesemiconductor layer 51, a second n-type nitride semiconductor layer 52,and a third n-type nitride semiconductor layer 53. These n-type nitridesemiconductor layers may be layers made of a nitride semiconductorcontaining n-type impurities such as Si or Ge. A nitride semiconductorconstituting these n-type nitride semiconductor layers may be, forexample, GaN and may include In and/or Al.

An n-type nitride semiconductor layer included in the first layeredportion 20 has an n-contact surface on which no additional semiconductorlayer is provided. An n-side electrode 81 is provided on the surface ofthe n-contact surface. The n-side electrode 81 is electrically connectedto the n-type nitride semiconductor layers included in the first layeredportion 20. The p-side electrode 82 is provided on the surface of thethird layered portion 50. The p-side electrode 82 is electricallyconnected to the third n-type nitride semiconductor layer 53 of thethird layered portion 50. That is, the n-side electrode 81 iselectrically connected to a semiconductor layer located on one side ofthe light emitting layer 30, and the p-side electrode 82 is electricallyconnected to a semiconductor layer located on the other side of lightemitting layer 30.

A forward voltage is applied between the p-side electrode 82 and then-side electrode 81. At this time, by applying the forward voltagebetween the p-type nitride semiconductor layers of the second layeredportion 40 and the n-type nitride semiconductor layers of the firstlayered portion 20, and by supplying electrons to the light emittinglayer 30, the light emitting layer 30 emits light.

When a positive potential is applied to the p-side electrode 82 and alower potential (e.g., negative potential) than the p-side electrode 82is applied to the n-side electrode 81, a reverse voltage is appliedbetween the p-type nitride semiconductor tunnel junction layer 43 andthe first n-type nitride semiconductor layer 51. Therefore, a currentbetween the p-type nitride semiconductor tunnel junction layer 43 andthe first n-type nitride semiconductor layer 51 uses a tunneling effect.That is, by tunneling electrons present in the valence band of thep-type nitride semiconductor tunnel junction layer 43 to the conductionband of the first n-type nitride semiconductor layer 51, a current iscaused to flow.

In order to obtain such a tunneling effect, a pn junction is formed bythe p-type nitride semiconductor tunnel junction layer 43 doped with ap-type impurity at a high concentration and the first n-type nitridesemiconductor layer 51 doped with an n-type impurity at a highconcentration. As the concentration of each conductive impuritycontained in the first n-type nitride semiconductor layer 51 and thep-type nitride semiconductor tunnel junction layer 43 increases, thewidth of a depletion layer formed at the interface between the p-typenitride semiconductor tunnel junction layer 43 and the first n-typenitride semiconductor layer 51 can be made narrower. Then, as the widthof the depletion layer becomes narrower, when a voltage is applied,electrons in the valence band of the p-type nitride semiconductor tunneljunction layer 43 easily tunnel the depletion layer and move to theconduction band of the first n-type nitride semiconductor layer 51.

For example, the Si concentration of the first n-type nitridesemiconductor layer 51 including Si as an n-type impurity is 5×10¹⁹/cm³or more and 2×10²¹/cm³ or less. For example, the Mg concentration of thep-type nitride semiconductor tunnel junction layer 43 including Mg as ap-type impurity is 1×10²⁰/cm³ or more and 5×10²¹/cm³ or less. Forexample, the width of the depletion layer formed by the p-type nitridesemiconductor tunnel junction layer 43 and the first n-type nitridesemiconductor layer 51 is 5 nm or more and 8 nm or less.

FIG. 2 is a flowchart illustrating a method of manufacturing a nitridesemiconductor device according to the present embodiment. As illustratedin FIG. 2, a method of manufacturing a nitride semiconductor deviceaccording to the present embodiment includes a p-type nitridesemiconductor tunnel junction layer forming step S103, a first n-typenitride semiconductor layer forming step S104, a second n-type nitridesemiconductor layer forming step S105, and a third n-type nitridesemiconductor layer forming step S106. The method of manufacturing anitride semiconductor device can further include, prior to step S103, aone or more n-type nitride semiconductor layers forming step S101 and alight emitting layer forming step S102. FIG. 3A to FIG. 3F are schematiccross-sectional views illustrating a method of manufacturing a nitridesemiconductor device 1 according to the present embodiment.

Each of the aforementioned nitride semiconductor layers contained in thenitride semiconductor device 1 is epitaxially grown over the substrate10 by a metal organic chemical vapor deposition (MOCVD) method in afurnace where the pressure and the temperature can be adjusted. Eachnitride semiconductor layer can be formed by introducing a carrier gasand a raw material gas into a furnace. As the carrier gas, a hydrogen(H₂) gas or a nitrogen (N₂) gas can be used. As a raw material gas of Nsource, an ammonia (NH₃) gas can be used. As a raw material gas of Gasource, a trimethylgallium (TMG) gas or a triethylgallium (TEG) gas canbe used. As a raw material gas of In source, a trimethylindium (TMI) gascan be used. As a raw material gas of Al source, a trimethylaluminum(TMA) gas can be used. As a raw material gas of Si source, a monosilane(SiH₄) gas can be used. As a raw material gas of Mg source, abiscyclopentadienylmagnesium (Cp₂Mg) can be used.

First, one or more n-type nitride semiconductor layers are formed on orabove the substrate 10 (step S101). As illustrated in FIG. 3A, a firstlayered portion 20 having one or more n-type nitride semiconductorlayers can be formed. As the first layered portion 20, a GaN layer 21,an n-type GaN layer 22, and a multilayer film 23 are layered in thisorder. Before forming the GaN layer 21 over the substrate 10, a bufferlayer may be formed on the surface of the substrate 10.

Next, as illustrated in FIG. 3B, a light emitting layer 30 is formed onor above one or more n-type nitride semiconductor layers (step S102).

Next, a p-type nitride semiconductor tunnel junction layer 43 is formedor above the light emitting layer 30 (step S103). As illustrated in FIG.3C, a second layered portion 40 including the p-type nitridesemiconductor tunnel junction layer 43 can be formed on the lightemitting layer 30. A p-type AlGaN layer 41, a GaN layer 42, and thep-type nitride semiconductor tunnel junction layer 43 are layered inthis order as the second layered portion 40. As the p-type nitridesemiconductor tunnel junction layer 43, for example, a GaN layer inwhich Mg is doped at a concentration of 2×10²⁰/cm³ is formed with athickness of 20 nm.

The p-type impurity concentration and the thickness of the p-typenitride semiconductor tunnel junction layer 43 are set to be able toform a tunnel junction with the first n-type nitride semiconductor layer51, which will be described later below. The p-type impurityconcentration of the p-type nitride semiconductor tunnel junction layer43 can be 5×10¹⁹/cm³ or more and 1×10²¹/cm³ or less. The p-type impurityconcentration of the p-type nitride semiconductor tunnel junction layer43 can be higher than the p-type impurity concentration of every layerbetween the light emitting layer 30 and the p-type nitride semiconductortunnel junction layer 43. The thickness of the p-type nitridesemiconductor tunnel junction layer 43 can be 2 nm or more and 30 nm orless. A nitride semiconductor constituting the p-type nitridesemiconductor tunnel junction layer 43 may be, for example, GaN, InGaN,or AlGaN. The nitride semiconductor constituting the p-type nitridesemiconductor tunnel junction layer 43 can be, for example, GaN.

The temperature in the step of forming each nitride semiconductor layerafter the step of forming the light emitting layer 30 is preferablyrelatively low, in consideration of the effect on the light emittinglayer 30. In a case in which the light emitting layer 30 includes anInGaN layer, the temperature in the subsequent step of forming eachnitride semiconductor layer is preferably less than 1000° C., is morepreferably less than or equal to 950° C., and may be greater than orequal to 800° C. The temperature in the step of forming each nitridesemiconductor layer can be the temperature of the substrate 10. Afterforming the p-type nitride semiconductor tunnel junction layer 43, thewafer composed of the substrate 10 and each semiconductor layer may betemporarily taken out from the furnace. After cleaning the wafer takenout from the furnace, the wafer may be returned into the reactor of thefurnace.

Next, as illustrated in FIG. 3D, a first n-type nitride semiconductorlayer 51 is formed on or above the p-type nitride semiconductor tunneljunction layer 43 (step S104). The first n-type nitride semiconductorlayer 51 forms a tunnel junction with the p-type nitride semiconductortunnel junction layer 43. The first n-type nitride semiconductor layer51 has a first n-type impurity concentration and a first thickness. Thefirst n-type nitride semiconductor layer 51 can be formed in contactwith the p-type nitride semiconductor tunnel junction layer 43.

The first n-type nitride semiconductor layer 51 is formed by introducinga carrier gas and a raw material gas into a furnace. As the carrier gas,a nitrogen gas or a hydrogen gas can be used. The temperature in thestep of forming the first n-type nitride semiconductor layer 51 ispreferably less than 1000° C., is more preferably less than or equal to950° C., and may be 800° C. or more. As the first n-type nitridesemiconductor layer 51, for example, a GaN layer doped with Si at aconcentration of 1×10²⁰/cm³ is formed with a thickness of 30 nm.

The first n-type impurity concentration and the first thickness of thefirst n-type nitride semiconductor layer 51 are set to be able to form atunnel junction with the p-type nitride semiconductor tunnel junctionlayer 43. For example, the tunnel junction can be made possible by thefirst n-type impurity concentration and the thickness described below.The first n-type impurity concentration can be higher than the secondn-type impurity concentration of the second n-type nitride semiconductorlayer 52 and can be higher than the third n-type impurity concentrationof the third n-type nitride semiconductor layer 53. The first n-typeimpurity concentration can be 1×10¹⁹/cm³ or more. The first n-typeimpurity concentration can be 2×10²¹/cm³ or less. For example, the Siconcentration of the first n-type nitride semiconductor layer 51including Si as an n-type impurity is 5×10¹⁹/cm³ or more and 2×10²¹/cm³or less. The first thickness can be 2 nm or more and 30 nm or less.

The nitride semiconductor constituting the first n-type nitridesemiconductor layer 51 is, for example, GaN, InGaN, or AlGaN. Thenitride semiconductor constituting the first n-type nitridesemiconductor layer 51 can be GaN or AlGaN. Thereby, the absorption oflight from the light emitting layer 30 can be reduced in comparison to acase where the nitride semiconductor constituting the first n-typenitride semiconductor layer 51 is InGaN. The nitride semiconductorconstituting the first n-type nitride semiconductor layer 51 may be GaN.Because the first n-type nitride semiconductor layer 51 contains ann-type impurity at a relatively high concentration, there is concernabout deterioration in the crystallinity, but the degree ofdeterioration in the crystallinity can be reduced with GaN.

Next, as illustrated in FIG. 3E, a second n-type nitride semiconductorlayer 52 is formed on or above the first n-type nitride semiconductorlayer 51 (step S105). The second n-type nitride semiconductor layer 52has a second n-type impurity concentration and a second thickness.

The second n-type nitride semiconductor layer 52 is formed byintroducing a carrier gas and a raw material gas into a furnace. Thesecond n-type nitride semiconductor layer 52 is formed in a nitrogenatmosphere. The term “nitrogen atmosphere” in the present disclosurerefers to a concentration of N₂, in the gas introduced into the furnace,being greater than or equal to 99.9% by volume. Also, as will bedescribed later, when H₂ is present, because the lateral growth ofnitride semiconductor is promoted, it is preferable that theconcentration of H₂ in the gas introduced into the furnace is less thanor equal to 0.01% by volume. For example, by using a nitrogen gas as thecarrier gas, it is possible to make the nitrogen atmosphere in thefurnace. The temperature in the step of forming the second n-typenitride semiconductor layer 52 is preferably less than 1000° C., is morepreferably 950° C. or less, and can be 800° C. or more. It is preferablethat the temperature in the step of forming the second n-type nitridesemiconductor layer 52 is equal to or less than the temperature in thestep of forming the first n-type nitride semiconductor layer 51. This isbecause the second n-type impurity concentration is less than the firstn-type impurity concentration. The higher the impurity concentration,the worse the crystallinity and the surface morphology, whereas thehigher the temperature at the time of formation, the better thecrystallinity and the morphology. Thus, the second n-type nitridesemiconductor layer 52 may be grown at a lower temperature than thefirst n-type nitride semiconductor layer 51, thereby reducing thepossibility of deterioration in the characteristics of the lightemitting layer 30. The temperature may be gradually increased from thestep of forming the first n-type nitride semiconductor layer 51 to thestep of forming the third n-type nitride semiconductor layer 53 thatwill be described later. As the second n-type nitride semiconductorlayer 52, for example, a GaN layer doped with Si at a concentration of5×10¹⁸/cm³ is formed with a thickness of 20 nm.

The second n-type impurity concentration of the second n-type nitridesemiconductor layer 52 is less than the first n-type impurityconcentration of the first n-type nitride semiconductor layer 51. Thus,the crystallinity of the second n-type nitride semiconductor layer 52can be enhanced, and the flatness of the third n-type nitridesemiconductor layer 53 to be subsequently grown can be enhanced. Thesecond n-type impurity concentration can be 1×10¹⁸/cm³ or more. Byintentionally doping an n-type impurity to the second n-type nitridesemiconductor layer 52, the resistance of the second n-type nitridesemiconductor layer 52 can be reduced and therefore the drive voltage ofthe obtained nitride semiconductor device 1 can be reduced. The secondn-type impurity concentration can be less than the first n-type impurityconcentration and can be less than or equal to 1×10²⁰/cm³. For example,the Si concentration of the second n-type nitride semiconductor layer 52including Si as an n-type impurity is 1×10¹⁸/cm³ or more and 1×10²⁰/cm³or less. The second thickness may be less than the first thickness. Thesecond thickness can be 10 nm or more and may be 40 nm or less.

The nitride semiconductor constituting the second n-type nitridesemiconductor layer 52 is, for example, GaN, InGaN, or AlGaN. Thenitride semiconductor constituting the second n-type nitridesemiconductor layer 52 is preferably GaN or AlGaN, and is morepreferably GaN. With the second n-type nitride semiconductor layer 52being an n-type GaN layer, the crystallinity of the third n-type nitridesemiconductor layer 53 to be subsequently formed can be enhanced.

Next, as illustrated in FIG. 3F, a third n-type nitride semiconductorlayer 53 is formed on or above the second n-type nitride semiconductorlayer 52 (step S106). The third n-type nitride semiconductor layer 53has a third n-type impurity concentration and a third thickness. Thefirst n-type nitride semiconductor layer 51, the second n-type nitridesemiconductor layer 52, and the third n-type nitride semiconductor layer53 can be formed continuously.

The third n-type nitride semiconductor layer 53 is formed by introducinga carrier gas and a raw material gas into a furnace. The third n-typenitride semiconductor layer 53 is formed in a hydrogen atmosphere. Theterm “hydrogen atmosphere” in the present disclosure refers to aconcentration of H₂, in the gas introduced into the furnace, being 60%by volume or more. For example, by using a hydrogen gas as the carriergas, it is possible to make the hydrogen atmosphere in the furnace. Thetemperature in the step of forming the third n-type nitridesemiconductor layer 53 is preferably less than 1000° C., is morepreferably 950° C. or less, and can be 800° C. or more. It is preferablethat the temperature in the step of forming the third n-type nitridesemiconductor layer 53 is equal to or less than the temperature in thestep of forming the first n-type nitride semiconductor layer 51.Thereby, the possibility of deterioration of the characteristics of thelight emitting layer 30 can be reduced. The temperature in the step offorming the third n-type nitride semiconductor layer 53 can be equal toor greater than the temperature in the step of forming the second n-typenitride semiconductor layer 52. As the third n-type nitridesemiconductor layer 53, for example, a GaN layer doped with Si at aconcentration of 5×10¹⁸/cm³ is formed with a thickness of 80 nm.

The third n-type impurity concentration of the third n-type nitridesemiconductor layer 53 is less than the first n-type impurityconcentration of the first n-type nitride semiconductor layer 51.Thereby, it is possible to enhance the flatness of the third n-typenitride semiconductor layer 53. By intentionally doping an n-typeimpurity to the third n-type nitride semiconductor layer 53, theresistance of the third n-type nitride semiconductor layer 53 can bereduced and therefore the drive voltage of the obtained nitridesemiconductor device 1 can be reduced. The third n-type impurityconcentration can be 1×10¹⁸/cm³ or more. The third n-type impurityconcentration can be less than the first n-type impurity concentrationand can be 1×10²⁰/cm³ or less. For example, the Si concentration of thethird n-type nitride semiconductor layer 53 including Si as an n-typeimpurity is 1×10¹⁸/cm³ or more and 1×10²⁰/cm³ or less.

The third thickness can be 10 nm or more and 500 nm or less. The thirdthickness of the third n-type nitride semiconductor layer 53 may begreater than the second thickness of the second n-type nitridesemiconductor layer 52. Thereby, it is possible to enhance the flatnessof the third n-type nitride semiconductor layer 53. For similar reasons,the third thickness may be greater than the first thickness.

The nitride semiconductor constituting the third n-type nitridesemiconductor layer 53 is, for example, GaN, InGaN, or AlGaN. Thenitride semiconductor constituting the third n-type nitridesemiconductor layer 53 is preferably GaN or AlGaN, and is morepreferably GaN. By the third n-type nitride semiconductor layer 53 beingan n-type GaN layer, the crystallinity can be enhanced.

A portion of the semiconductor layered body is then removed to expose aportion of the first layered portion 20. Then, as illustrated in FIG. 1,the p-side electrode 82 is formed on the third layered portion 50, andthe n-side electrode 81 is formed on the exposed surface of the firstlayered portion 20. As the n-side electrode 81 and the p-side electrode82, translucent electrodes 81A and 82A such as ITO electrodes can beformed first, and then metal electrodes 81B and 82B can be formed.Thereby, the nitride semiconductor device 1 illustrated in FIG. 1 can beobtained.

Another semiconductor layer may be further formed on the third n-typenitride semiconductor layer 53. FIG. 4 is a schematic cross-sectionalview of a nitride semiconductor device 2 according to a modifiedexample. FIG. 5 is a flowchart illustrating a method of manufacturingthe nitride semiconductor device 2 according to the modified example. Asillustrated in FIG. 4, the nitride semiconductor device 2 includes afirst light 217, emitting layer 30A and a second light emitting layer30B. The p-type nitride semiconductor tunnel junction layer 43 and thefirst n-type nitride semiconductor layer 51 are arranged at positionsinterposed between the first light emitting layer 30A and the secondlight emitting layer 30B. A fourth layered portion 60 including one ormore p-type nitride semiconductor layers is arranged on the second lightemitting layer 30B. A p-side electrode 82 is connected to the fourthlayered portion 60. As illustrated in FIG. 5, a method of manufacturingthe nitride semiconductor device 2 includes a step S107 of forming thesecond light emitting layer 30B on or above the third n-type nitridesemiconductor layer 53, and a step S108 of forming one or more p-typenitride semiconductor layers on or above the second light emitting layer30B.

The peak wavelength of light emission of the first light emitting layer30A and the peak wavelength of light emission of the second lightemitting layer 30B may be the same or different. The light emission peakwavelength of the first light emitting layer 30A and the light emissionpeak wavelength of the second light emitting layer 30B are, for example,430 nm or more and 540 nm or less. The first and second light emittinglayers 30A and 30B emit blue light or green light. By layering thesecond light emitting layer 30B above the first light emitting layer30A, the output per unit area can be increased in comparison to a lightemitting device including a single light emitting layer.

In a case in which a tunnel junction is used, as in the nitridesemiconductor device 1 according to the present embodiment, a nitridesemiconductor layer of which the impurity concentration is relativelyhigh is formed. For such a nitride semiconductor layer, because theimpurity concentration is high, the flatness tends to deteriorate. Also,because the nitride semiconductor layer is formed after the lightemitting layer 30, the nitride semiconductor layer is preferably formedat a low temperature at which the light emitting layer 30 does notdeteriorate due to thermal damage, and the flatness tends to deterioratefrom this point as well. According to the present embodiment, by formingthe second n-type nitride semiconductor layer 52 and the third n-typenitride semiconductor layer 53 subsequent to the first n-type nitridesemiconductor layer 51, the flatness of the third n-type nitridesemiconductor layer 53 can be enhanced. The second n-type nitridesemiconductor layer 52 is formed in a nitrogen atmosphere and the thirdn-type nitride semiconductor layer 53 is formed in a hydrogenatmosphere. Due to the difference in the atmosphere at the time offormation, the second n-type nitride semiconductor layer 52 undergoesthree-dimensional crystal growth, and the third n-type nitridesemiconductor layer 53 undergoes crystal growth that promotes lateralgrowth. By growing the second n-type nitride semiconductor layer 52 onor above the rough morphology of the first n-type nitride semiconductorlayer 51, it is presumed that three-dimensional crystal growth isperformed on or above an uneven portion of the rough morphology at afiner scale, thereby reducing a large uneven portion. As a result, it isconsidered that flattening is easily made at the time of performinggrowth in such a manner that an uneven portion is embedded by thelateral growth by the third n-type nitride semiconductor layer 53 to besubsequently formed. In this manner, by enhancing the flatness of thethird n-type nitride semiconductor layer 53, a decrease in the withstandvoltage characteristic and the occurrence of leakage caused bydeterioration of the flatness of the third n-type nitride semiconductorlayer 53 can be suppressed. Thereby, a nitride semiconductor device withenhanced reliability can be obtained.

As an experimental example, up to the third n-type nitride semiconductorlayer 53 was formed, the surface was observed by an atomic forcemicroscope (AFM). In the experimental example, a GaN layer having Sidoped at a concentration of 1×10²⁰/cm³ was formed with a thickness of 30nm as the first n-type nitride semiconductor layer 51, a GaN layerhaving Si doped at a concentration of 5×10¹⁸/cm³ was formed with athickness of 20 nm as the second n-type nitride semiconductor layer 52,and a GaN layer having Si doped at a concentration of 5×10¹⁸/cm³ wasformed with a thickness of 80 nm as the third n-type nitridesemiconductor layer 53. Also, as a comparative example, with theexception of forming a GaN layer doped with Si at a concentration of5×10¹⁸/cm³ at a thickness of 100 nm without forming the second n-typenitride semiconductor layer 52, each nitride semiconductor layer wasformed in the same manner as the above described experimental example,and an AFM image of the surface of the Si-doped GaN layer was observed.

FIG. 6 illustrates an AFM observation image of the experimental exampleand FIG. 7 illustrates an AFM observation image of the comparativeexample. Each of FIG. 6 and FIG. 7 is an observation image of a squarearea having a side of 2.0 μm. As illustrated in FIG. 6 and FIG. 7, thesurface state for which the second n-type nitride semiconductor layer 52was formed had fewer pits. That is, it was found that the flatness ofthe third n-type nitride semiconductor layer 53 can be enhanced byforming the second n-type nitride semiconductor layer 52. Thus, byenhancing the flatness of the third n-type nitride semiconductor layer53, a nitride semiconductor device with increased reliability can beobtained.

Embodiments of the present disclosure have been described above withreference to specific examples. However, the present disclosure is notlimited to these examples. Any embodiment that can be implemented by aperson skilled in the art modifying the above described embodiment ofthe present disclosure is encompassed by the scope of the presentdisclosure as long as encompassing the gist of the present disclosure.In addition, within the idea of the present disclosure, a person skilledin the art may come up with various changes and modifications, and it isunderstood that these changes and modifications also belong to the scopeof the present disclosure.

A method of manufacturing a nitride semiconductor device according tothe present disclosure can be used in various fields using nitridesemiconductor devices such as lighting fixtures, vehicle-mountedlighting, and displays.

What is claimed is:
 1. A method of manufacturing a nitride semiconductordevice, the method comprising: forming, on or above a p-type nitridesemiconductor tunnel junction layer, a first n-type nitridesemiconductor layer that forms a tunnel junction with the p-type nitridesemiconductor tunnel junction layer, the first n-type nitridesemiconductor layer having a first impurity concentration and a firstthickness; forming, on or above the first n-type nitride semiconductorlayer, in a nitrogen atmosphere, a second n-type nitride semiconductorlayer having a second n-type impurity concentration less than the firstn-type impurity concentration and a second thickness; and forming, on orabove the second n-type nitride semiconductor layer, in a hydrogenatmosphere, a third n-type nitride semiconductor layer having a thirdn-type impurity concentration less than the first n-type impurityconcentration and a third thickness.
 2. The method according to claim 1,wherein the third thickness is greater than the second thickness.
 3. Themethod according to claim 1, wherein the second thickness is 10 nm ormore.
 4. The method according to claim 1, wherein the first n-typeimpurity concentration is 1×10¹⁹/cm³ more.
 5. The method according toclaim 1, wherein each of the second n-type impurity concentration andthe third n-type impurity concentration is 1×10¹⁸/cm³ or more.
 6. Themethod according to claim 1, wherein a nitride semiconductorconstituting the first n-type nitride semiconductor layer and the secondn-type nitride semiconductor layer is GaN or AlGaN.
 7. The methodaccording to claim 1, wherein a temperature in forming the second n-typenitride semiconductor layer is less than or equal to a temperature informing the first n-type nitride semiconductor layer.
 8. The methodaccording to claim 1, wherein a temperature in forming the third n-typenitride semiconductor layer is equal to or higher than a temperature informing the second n-type nitride semiconductor layer.
 9. The methodaccording to claim 1, further comprising, prior to forming the firstn-type nitride semiconductor layer: forming one or more n-type nitridesemiconductor layers on or above a substrate; forming a light emittinglayer on or above the one or more n-type nitride semiconductor layers;and forming the p-type nitride semiconductor tunnel junction layer on orabove the light emitting layer.
 10. The method according to claim 9,wherein the light emitting layer is a first light emitting layer,wherein the method comprises, after the forming of the third n-typenitride semiconductor layer: forming a second light emitting layer on orabove the third n-type nitride semiconductor layer; and forming one ormore p-type nitride semiconductor layers on or above the second lightemitting layer.
 11. The method according to claim 1, wherein the firstn-type nitride semiconductor layer is formed in contact with the p-typenitride semiconductor tunnel junction layer.
 12. The method according toclaim 1, wherein a nitride semiconductor constituting the p-type nitridesemiconductor tunnel junction layer is GaN, InGaN or AlGaN.
 13. Themethod according to claim 1, wherein the third thickness is greater thanthe first thickness.
 14. A method of manufacturing a nitridesemiconductor device, the method comprising: forming one or more n-typenitride semiconductor layers on or above a substrate; forming a firstlight emitting layer on or above the one or more n-type nitridesemiconductor layers; forming, on or above the first light emittinglayer, a p-type nitride semiconductor tunnel junction layer having ap-type impurity concentration of 5×10¹⁹/cm³ or more and 1×10²¹/cm³ orless and a thickness of 2 nm or more and 30 nm or less; forming, on thep-type nitride semiconductor tunnel junction layer, a first n-typenitride semiconductor layer having a first impurity concentration of1×10¹⁹/cm³ or more and a first thickness of 2 nm or more and 30 nm orless; forming, on or above the first n-type nitride semiconductor layer,in a nitrogen atmosphere, a second n-type nitride semiconductor layerhaving a second n-type impurity concentration less than the first n-typeimpurity concentration and a second thickness; forming, on or above thesecond n-type nitride semiconductor layer, in a hydrogen atmosphere, athird n-type nitride semiconductor layer having a third n-type impurityconcentration less than the first n-type impurity concentration and athird thickness; forming a second light emitting layer on or above thethird n-type nitride semiconductor layer; and forming one or more p-typenitride semiconductor layers on or above the second light emittinglayer.
 15. The method according to claim 14, wherein the third thicknessis greater than the second thickness.
 16. The method according to claim14, wherein the second thickness is 10 nm or more.
 17. The methodaccording to claim 14, wherein each of the second n-type impurityconcentration and the third n-type impurity concentration is 1×10¹⁸/cm³or more.
 18. The method according to claim 14, wherein a nitridesemiconductor constituting the first n-type nitride semiconductor layerand the second n-type nitride semiconductor layer is GaN or AlGaN. 19.The method according to claim 14, wherein a nitride semiconductorconstituting the p-type nitride semiconductor tunnel junction layer isGaN, InGaN or AlGaN.
 20. The method according to claim 14, wherein eachof the first light emitting layer and the second light emitting layer isconfigured to emit light having a peak wavelength of 430 nm or more and540 nm or less.